The present invention relates generally to alloys and to shaped articles formed for structural use at high temperatures. More particularly, it relates to an alloy having a niobium base and which contains hafnium and aluminum additives. By a niobium base is meant that the principal ingredient of the alloy is niobium.
There are a number of uses for metals which have high strength at high temperature. One particular attribute of the present invention is that it has, in addition to high strength at high temperature, a relatively lower density of the order of 7.8 to 8.8 grams per cubic centimeter (g/cc).
In the field of high temperature alloys and particularly alloys displaying high strength at high temperature, there are a number of concerns which determine the field applications which can be made of the alloys. One such concern is the compatibility of an alloy in relation to the environment in which it must be used. Where the environment is the atmosphere, this concern amounts to a concern with the oxidation or resistance to oxidation of the alloy.
Another such concern is the density of the alloy. One of the groups of alloys which is in common use in high temperature applications is the group of iron-base, nickel-base, and cobalt-base superalloys. The term "base", as used herein, indicates the primary ingredient of the alloy is iron, nickel, or cobalt, respectively. These superalloys have relatively high densities of the order of 8 to 9 g/cc. Efforts have been made to provide alloys having high strength at higher operating temperatures and significantly above those of the superalloys.
It has been observed that the mature metal candidates for use in this field of high strength at high temperature can be grouped and such a grouping is graphically illustrated in FIG. 1. Referring now to FIG. 1, the ordinate of the plot shown there is the density of the alloy and the abscissa is the maximum temperature at which the alloy provides useful structural properties for aircraft engine applications. The prior art alloys in this plot are discussed in descending order of density and use temperatures.
With reference to FIG. 1, the materials of highest density and highest use temperatures are those enclosed within an envelope marked as Nb-base and appearing in the upper right hand corner of the figure. Densities range from about 8.7 to about 9.7 grams per cubic centimeter and use temperatures range from less than 2200.degree. F. to about 2600.degree. F.
Referring again to FIG. 1, the group of prior art iron, nickel, and cobalt based superalloys are seen to have the next highest density and also a range of temperatures at which they can be used extending from about 500.degree. F. to about 2200.degree. F.
A next lower density group of prior art alloys are the titanium-base alloys. As is evident from the figure, these alloys have a significantly lower density than the superalloys but also have a significantly lower set of use temperatures ranging from about 200.degree. F. to about 900.degree. F.
The last and lowest density group of prior art alloys are the aluminum-base alloys. As is evident from the graph these alloys generally have significantly lower density. They also have relatively lower temperature range in which they can be used, because of their low melting points.
The usefulness of the titanium-base alloys extends over a temperature range which is generally higher than that of the aluminum-base alloys but lower than that of the superalloys. Within this temperature range, alloy changes occur due to a phase transformation from hexagonal to cubic crystal structure.
A novel additional set of alloys is illustrated in the figure as having densities about equal to those of the superalloys but with useful temperature ranges potentially extending beyond the superalloy temperature range. These ranges of temperature and density include those for the alloys such as are provided by the present invention and which are formed with a niobium base.